Ocular Pharmacology Flashcards
What are the 3 ways a drugs is administered topically is distributed in the eye?
- Drained by lacrimal drainage
- Penetrates the cornea/sclera
- Enters systemic circulation through conjunctival and nasal mucosa vessels
What % of the tear film is turned over every minute?
15%
What are the advantages of topical administration into the eye?
- Convenient
- Easy
- Specific
- Minimal side effects
What needs to be considered when topically administering a drug into the eye?
Intraocular bioavailability is relatively poor - 1-10% of instilled drug reaches the anterior chamber
Palpebral fissure can only hold about 20μL – dropper bottle delivers 50μL/drop
How can intraocular bioavailability be improved?
- Increasing retention time on ocular surface
- Optimising the ability of the drug to penetrate the cornea
What are the different formulas a drug can come in for ocular treatment?
- solution
- suspension
- ointment
- viscous gel
- solid delivery
- sub-conjunctival injection
Most preparations are formulated to enter the eye through which part?
Cornea
How do drugs move through the epithelium layer of the cornea?
Paracellularly - between cells (water soluble)
Transcellular - through cells (lipid soluble)
The stroma of the cornea facilitates the diffusion of which drug types?
Water soluble
Which layer of the cornea can both lipid and water soluble drugs pass through with ease?
Endothelium
To be able to penetrate the cornea a drug should have what characteristic?
Hydrophilic and lipophilic
Are ionised forms water or lipid soluble?
Water soluble (unionised are lipid soluble)
Do ionised or unionised forms cross the corneal epithelium more easily?
Unionised
Once in the stroma what happens to the unionised drug?
It becomes ionised because of its relative abundance, which facilitates its movement across the stroma
Which action of a drug can render it unavailable?
Binding to the tear film
How do large hydrophilic molecules enter the eyeball?
Absorption across the conjunctiva and sclera
Once a drug penetrates the cornea where does it move into?
Aqueous humour, iris and ciliary body
If a drug is administered via a non-corneal route where are its levels highest?
Anterior uvea
How do the levels of being lipophilic change for a drug treating more superficially e.g. cornea vs deeper structures e.g. anterior uvea
Superficial = less lipophilic Deeper = more lipophilic
The Blood:ocular barrier consists of?
The blood:aqueous and the blood:retinal barriers
Why can agents cross more easily into the anterior segment than the posterior?
The blood:aqueous barrier is leakier than the blood:ocular barrier so systemic agents can cross more easily into anterior segment
Give some examples of when we would want to change the pupil size
- Dilate the pupil to enable fundic examination
- Relax ciliary body to relieve spasm
- Constrict pupil to open drainage angle
What is the proper word for dilation of the pupil?
Mydriasis
What is the proper word for relaxation of the ciliary body?
Cycloplegia
